Disc drive units in general are known in the art for use in modern microcomputers such as personal and desk top computers. In the most common form, such disc drive units comprise a so-called Winchester disc drive having one or more rotatable memory storage discs mounted within a substantially sealed disc drive housing along with one or more related electromagnetic heads for reading and writing data on appropriately prepared disc surfaces. A disc drive of this type is sometimes referred to as a hard or a fixed disc drive and is normally available as a compact, complete package with the sealed housing on a rigid chassis or frame together with a circuit board carrying the necessary drive electronics.
Within the sealed disc drive, a rotating constant speed spindle motor is provided for mounting and rotating the discs. Because of the extremely fine tolerances in the internal components to the disc drive especially the head and the disc surface, the head disc assembly is sealed against the outside air to prevent the entry of contaminants. In order to minimize the size of the disc drive, modern spindle motors are designed with many components inside the hub which supports the discs. Therefore, obviously, it is also critical to provide a seal between the spindle motor which mounts the discs for rotation and the interior of the head disc assembly. Magnetic fluid seals are already widely used in spindle motors for disc drives as a means to seal either the external components from the inside of the drive, thus preventing external contaminants such as dust particles from entering the head disc assembly; or are used to seal portions of the regions within the head disc assembly to prevent the flow of air through the motor bearings, thus preventing particles such as grease aerosols from circulating from the motor into the interior of the head disc assembly.
It is standard practice in the disc drive industry in using magnetic fluid seals to use an adhesive to retain the seal in its defined location, and to prevent airflow around the outside diameter of the seal. A typical example of such a device shown in the simplest form appears in FIG. 1 and includes a housing 1 and shaft 3. A pair of annular pole pieces 13 and 15 are provided, sandwiching a permanent magnet 11 which is integrally inserted therebetween to form the ferro-fluidic seal of the device. The ferro-fluid 9 is disposed between the pole pieces 13 and 15 and the shaft 3. The magnet 11 causes the ferro-fluid to be retained in place between the pole-pieces 13 and 15 and the shaft 3 so that a seal is formed through which contaminants cannot pass. As shown in this example, which is taken from U.S. Pat. No. 5,161,902, in order to prevent contaminants from passing through the outer radial gap G between the housing 1 and the pole pieces 13, 15, the gap between at least one of the pole pieces and the housing is filled with an adhesive bonding agent 23.
As disclosed in that patent and as is common practice in the industry, the bonding agent 23 serves to completely prevent foreign material from entering the inside space through the sealing device. As further disclosed in the prior art, without the bonding agent 23, foreign materials such as grease which have penetrated through small clearances between the housing 1 and the pole piece 15 and the magnet 11 could pass through the gap G into the inside space of the disc assembly, which must be kept clean. It is known to be highly desirable to eliminate the need for this adhesive because the requirements for the adhesive are particularly demanding, and pose a difficult manufacturing and design problem. The adhesive requirements include maintaining structural strength over temperature cycles of -40.degree. to +70.degree. C.; a fast and complete cure; low outgassing; and being made of an easy to dispense, single component. Very few practical adhesives are available to satisfy all these requirements. Even the adhesives that are available are expensive, and the cost of installation (process time plus tools) is high. Moreover, even the more expensive ones have failed to eliminate the problem of outgassing. Therefore efforts have been made to eliminate adhesives from the assembly of disc drive motors.